I feel like I understand communist theory pretty well at a basic level, and I believe in it, but I just don't see what part of it requires belief in an objective world of matter. I don't believe in matter and I'm still a communist. And it seems that in the 21st century most people believe in materialism but not communism. What part of "people should have access to the stuff they need to live" requires believing that such stuff is real? After all, there are nonmaterial industries and they still need communism. Workers in the music industry are producing something that nearly everyone can agree only exists in our heads. And they're still exploited by capital, despite musical instruments being relatively cheap these days, because capital owns the system of distribution networks and access to consumers that is the means of profitability for music. Spotify isn't material, it's a computer program. It's information. It's a thoughtform. Yet it's still a means of production that ought to be seized for the liberation of the musician worker. What does materialism have to do with any of this?
Idk if you're doing a bit, but assuming that you're serious and having read through some of your comments below, I think the problem that you're having is largely based in a misunderstanding of what people tend to mean when they argue that something is socially constructed. And, relatedly, that you're working from an opposition between the real and the imaginary that can't account for the complexity of their actual relation. Correct me if I'm wrong, but your basic assumption seems to be that if something is socially constructed then it is imaginary/'ideal' and, therefore, that isn't real/material. And, further, that if something is socially constructed then it emerges as a creation of the individual human mind.
The problem with the first assumption is that socially constructed forms are still real and material forms. Even what at first glance might appear as immaterial forms (such as the dominant ideas of a society or music) emerge from within a historical and material context that works to structure them and provide the conditions of possibility for certain ideas and forms to emerge, and these likewise operate back upon that context in real ways. To use your example of the commodities produced by the music industry, the apparent 'immateriality' of a song still depends upon a wide range of material forms. Among these are the material forms of the instruments used in its creation, the historical traditions of music and the material forms necessary for archiving and preserving them into the present of the song's production, the material networks that facilitate and determine the song's distribution (which include everything from the record companies that sign and promote artists, to the mines in which the raw materials that are used in the production of both the instruments being played and the computers and speakers upon which the song eventually comes to be heard are excavated). You can see here already that the relationship between the apparently 'ideal' and the 'material' is far more complex than a simple binary opposition.
This leads to the problem with the second assumption that you seem to be making, which is that you seem to be positing a genuinely idealist understanding of ideas and the human subject in which ideas emerge in the manner of a virgin birth from the individual human subject (this being the only form that would preserve their genuinely 'ideal' being from being muddied by a dirty materialism). The problem with this belief is that it fails to account for the historical production of that subject - a historical production which is, ironically, a key idea within social constructivist theories. Ideas are necessarily social forms, their existence implies intersubjectivity through the existence of language. In this sense, ideas necessarily have a material dimension that fatally undermines the kind of idealist conception that you seem to be expressing. This is because our individual subjectivity and thus our ability to have ideas emerges within a substrate that is outside and beyond us. We are already structured in certain ways as a condition of being able to think and this is the basis of a materialist understanding